As it is known, in the strip continuous casting with machine equipped with counter-rotating rolls, one of the main problems is constituted by the presence of impurities in the steel, which are typically constituted by oxides (coming from refining, transfer or casting of the molten metal) and by particles of refractory material (coming from the commonly utilized devices), which tend to float up and to conglomerate on the surface of the molten metal bath causing thickenings which can reach even a few squared-centimetre-wide area. Such impurities (commonly known under the term “scum”) come then into contact with the surface of the rotating rolls and are dragged therefrom, then solidifying on the surface of the strip which is going to solidify and thus creating defects on the surface of the strip itself.
From the state of the art some devices are known to avoid the arising of this problem: both trying to limitate the metal oxidation, for example by protecting the liquid bath in the mould with inert gas, and by producing an extremely clean molten metal. Nevertheless, in practice it is not possible, at least in an industrial process, avoiding an even minimum oxidation of the steel during the treatment or the transfer for example from the ladle to the tundish. Other polluting sources, as already said, are constituted by the refractory materials used, such as the tundish coating, the stopper rod or the discharger feeding the metal into the mould or the metal covering powders in the tundish.
For this reason systems have been developed, which usually utilize barriers dipped into the molten metal in the mould parallely to the rolls' axis, which tend to the mould in parallelin parallel to the rolls′ axis which tend to avoid the accumulation of these impurities near the surface of the casting rolls.
In particular, JP 6-106304 and JP 2001-321897 provide a pair of long barriers, arranged in parallelin parallel to the rolls' axis, dipped into the molten metal and a molten metal supply by means of a submersed discharger (hereinafter also designated as plunger) with holes directed towards the rolls' surface. A part of the molten metal flow strikes against the barriers, which are positioned between plunger and rolls, and it is reflected inwards, whereas a part thereof passes under the barriers and creates a flow parallel to the rolls' surface which drag the floating impurities confining them inside the compartment constituted by the barriers.
With this solution the floating impurities are not wholly moved away from the rolls' surface and in particular accumulations can form near the rolls in the bath area near the mould corners. This because the fluid flow directed towards the rolls is quite weak and not much effective since it is partially shielded by the barriers themselves and the holes are not directed so as to favour the movement of the impurities towards the mould corners. Furthermore, the superficial flow induced by the fluid part reflected by the barriers and directed towards the side plates, near the mould corners contrasts with the other superficial flow parallel to the rolls, hindering an easy accumulation of the impurities inside the barriers and creating stagnation areas between the barriers and the rolls where the impurities can thicken and where even steel solidifications may form with consequent defects on the cast strip surface.
Another drawback of this solution is that limited variations (already in the order of millimetres) in the positioning of the barriers or the holes of the nozzle cause significant variations in the metal flow which is rejected by the barriers and in that which passes thereunder and this significantly changes the fluiddynamic behaviour induced by the system. In an industrial iron-metallurgic context this constitutes a serious problem because, due to the dimensional tolerances, to the assembly techniques commonly used for the refractory components, to the unavoidable thermal expansions and to wears of the existing components, it is practically impossible to assure such a precision during the strip casting process.
Furthermore, the device disclosed in JP 6-106304 does not provide that at least a part of the molten metal supplied by the plunger be directly directed towards the side plates and this generally involves the formation of undesired solidifications on the plates themselves, with consequent serious casting problems.
The device disclosed in JP 2001-321897 utilizes a molten metal supply partially directed towards the side plates so that the discharger holes directed towards the side plates have a total area ranging between 0.3 and 0.7 of the total area of the other holes directed towards the surface of the casting rolls. On one side, if this solution avoids the formation of undesired solidifications on the side plates, on the other side it causes a fluid flow directed towards the plates which worsens the critical situation already described by opposing itself too to the superficial flow running parallel to the rolls' surface and it hinders an easy accumulation of the impurities inside the barriers.
U.S. Pat. No. 5,385,199 discloses the use of two barriers dipped into the molten metal bath parallel to the surface of the casting rolls at a distance from such surface ranging from 3 to 10 mm. In this case the purpose is to avoid the thickening of impurities on the bath surface near to the rolls by exploiting the turbulent motion which arises in the limited space between barrier and roll due to the revolving motion of the rolls themselves. However, with this solution steel solidifications can easily form on the bath between barriers and rolls. These solidifications impair the regular strip solidification causing unacceptable defects on the strip surface such as cracks and depressions.
Therefore, in the specific field there is the need of having at disposal an apparatus which to overcome the drawbacks inherent in the state of art.
This need is fulfilled by the present invention which furthermore has other advantages which will be evident as illustrated hereinafter.